Multilumen central venous catheters are commonly used in the treatment of critically ill patients to administer fluids and medications, monitor central venous pressure, and withdraw blood samples. The purpose of this randomized, comparison study is to determine if the method of blood withdrawal from multilumen central venous catheters has an effect on the accuracy of lab results and/or catheter function. Adult subjects who require periodic blood samples for laboratory studies are randomly assigned to one of three blood withdrawal methods: (1) vacutainer, (2) syringe, (3) in-line closed blood withdrawal system. Laboratory results obtained from each of these methods are compared to results obtained from a simultaneously obtained venipuncture specimen. Catheter function is evaluated by a hemodynamic wave form analysis via multivariate techniques. Preliminary analysis of 17 subjects reveals no difference among the three techniques on lab accuracy, catheter function and catheter patency.